import and use timegm from musl

It's ridiculous this function is not in the standards.

1 files changed, 86 insertions, 0 deletions

diff --git a/mytimegm.c b/mytimegm.c
new file mode 100644
index 0000000..3d3fccd
--- /dev/null
+++ b/mytimegm.c
@@ -0,0 +1,86 @@
+#include <time.h>
+
+// from musl@1cc81f5cb, slightly tweaked
+
+static long long
+__year_to_secs(long long year, int *is_leap)
+{
+        if (year-2ULL <= 136) {
+                int y = year;
+                int leaps = (y-68)>>2;
+                if (!((y-68)&3)) {
+                        leaps--;
+                        if (is_leap) *is_leap = 1;
+                } else if (is_leap) *is_leap = 0;
+                return 31536000*(y-70) + 86400*leaps;
+        }
+
+        int cycles, centuries, leaps, rem;
+
+        cycles = (year-100) / 400;
+        rem = (year-100) % 400;
+        if (rem < 0) {
+                cycles--;
+                rem += 400;
+        }
+        if (!rem) {
+                *is_leap = 1;
+                centuries = 0;
+                leaps = 0;
+        } else {
+                if (rem >= 200) {
+                        if (rem >= 300) centuries = 3, rem -= 300;
+                        else centuries = 2, rem -= 200;
+                } else {
+                        if (rem >= 100) centuries = 1, rem -= 100;
+                        else centuries = 0;
+                }
+                if (!rem) {
+                        *is_leap = 0;
+                        leaps = 0;
+                } else {
+                        leaps = rem / 4U;
+                        rem %= 4U;
+                        *is_leap = !rem;
+                }
+        }
+
+        leaps += 97*cycles + 24*centuries - *is_leap;
+
+        return (year-100) * 31536000LL + leaps * 86400LL + 946684800 + 86400;
+}
+
+static int
+__month_to_secs(int month, int is_leap)
+{
+        static const int secs_through_month[] = {
+                0, 31*86400, 59*86400, 90*86400,
+                120*86400, 151*86400, 181*86400, 212*86400,
+                243*86400, 273*86400, 304*86400, 334*86400 };
+        int t = secs_through_month[month];
+        if (is_leap && month >= 2) t+=86400;
+        return t;
+}
+
+time_t tm_to_secs(const struct tm *tm)
+{
+        int is_leap;
+        long long year = tm->tm_year;
+        int month = tm->tm_mon;
+        if (month >= 12 || month < 0) {
+                int adj = month / 12;
+                month %= 12;
+                if (month < 0) {
+                        adj--;
+                        month += 12;
+                }
+                year += adj;
+        }
+        long long t = __year_to_secs(year, &is_leap);
+        t += __month_to_secs(month, is_leap);
+        t += 86400LL * (tm->tm_mday-1);
+        t += 3600LL * tm->tm_hour;
+        t += 60LL * tm->tm_min;
+        t += tm->tm_sec;
+        return t;
+}